Mesoporous-silica films, fibers, and powders by evaporation

ABSTRACT

This invention pertains to surfactant-templated nanometer-scale porosity of a silica precursor solution and forming a mesoporous material by first forming the silica precursor solution into a preform having a high surface area to volume ratio, then rapid drying or evaporating a solvent from the silica precursor solution. The mesoporous material may be in any geometric form, but is preferably in the form of a film, fiber, powder or combinations thereof. The rapid drying or evaporation of solvent from the solution is accomplished by layer thinning, for example spin casting, liquid drawing, and liquid spraying respectively. Production of a film is by layer thinning, wherein a layer of the silica precursor solution is formed on a surface followed by removal of an amount of the silica precursor solution and leaving a geometrically thinner layer of the silica precursor solution from which the solvent quickly escapes via evaporation. Layer thinning may be by any method including but not limited to squeegeeing and/or spin casting. In powder formation by spray drying, the same conditions of fast drying exists as in spin-casting (as well as in fiber spinning) because of the high surface-area to volume ratio of the product. When a powder is produced by liquid spraying, the particles or micro-bubbles within the powder are hollow spheres with walls composed of mesoporous silica. Mesoporous fiber formation starts with a similar silica precursor solution but with an added pre-polymer making a pituitous mixture that is drawn into a thin strand from which solvent is evaporated leaving the mesoporous fiber(s).

This application is a continuation-in-part of U.S. application Ser. No.08/753,573 filed Nov. 26, 1996, now abandoned.

This invention was made with Government support under Contract DE-AC0676RLO 1830 awarded by the U.S. Department of Energy. The Government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to a mesoporous silica materialmade by an evaporative method. More specifically, the present inventionrelates to films, fibers and powders having mesoporous structure madefrom a silica precursor solution by layer thinning, for example spincasting; drawing; and spraying respectively. As used herein, the term“silica” means the presence of silicon (Si), without precludingadditional metals.

BACKGROUND OF THE INVENTION

Porous silica powders, with ordered porosity in the nanometer scale,have utility for catalysis, gas separation and high surface areasupports for self-assembled monolayer films. Mesoporous micro-bubbles inparticular, have applications in separations, thermal barriers andmicro-encapsulation for drug delivery.

Micron-sized bubbles composed of solid silica walls are commerciallyavailable and are used as fillers and within reflective paint forhighway signs. U.S. Pat. No. 2,797,201 (Standard Oil Co., Ohio)describes hollow glass spheres with solid walls, by spray drying liquidalkali metal silicates containing a blowing agent. Sizes range from50-300 μm. Because these products are not porous, they are not usefulfor catalysis, and gas separation.

Other formation techniques for mesoporous powders and films, discussedin the literature, involve slow growth from supersaturated solutions forseveral hours to one week. The previous methods are based on aprecipitation processes in which dissolved silica co-precipitates withthe surfactant micelles to form a mesoporous structure and typicallyinvolve heating the reactants in an autoclave for several hours to aweek. A disadvantage of these methods is that there is no control overparticle size and/or shape. Filtration, often tedious because of smallparticle size, is required to separate the solution from the mesoporousparticles.

Work described in U.S. Pat. Nos. 5,264,203, 5,098,684, 5,102,643, and5,238,676 shows mesoporous powder formation by in-situ solution-phaseprecipitation, which again requires substantial time from a minimum ofabout 1.25 hour to about 168 hour to obtain precipitated powders.

Huo et al., Chem. Mater. 1994, 6, 1176, discussed a method for producingmesoporous silica by an acid route. Tetraethoxysilane (TEOS) was addedto a dilute aqueous solution of cetyltrimethyl ammonium chloride (CTAC)and HCl. The solution composition on a mole basis was: TEOS 1.0; CTAC0.12; HCl 9.2; water 130. After ˜30 min of stirring at room temperature,particles precipitated and were filtered from the remaining solution.Again, a significant amount of time (30 min) is needed to obtainprecipitation of particles from the solution phase.

Tanev, P. T.; Pinnavaia, T. J.; Science, 1996, 271, 1267 used surfactantvesicles to template silica vesicles in a reaction mixture. The reactionmixture was vigorously stirred at ambient temperature for 18 hours toobtain the templated lamellar product with vesicular morphology, denotedMSU-V.

The method discussed by both Kresge, C. T., et al., Nature 1992, 359,710; and Beck, J. S., et al., J. Am. Chem. Soc. 1992, 114, 10834involves a slow growth, or co-precipitation, of silica and surfactantmicelles over a period of 4 hours to 144 hours (5 days). Beck, J. S.;Hellring, S. D.; Vartuli, J. C. Abstract # COLL-311, ACS NationalMeeting, April 13-17, San Francisco, Calif., 1997, further indicate that1700 m²/g is presently an upper limit of surface area.

Porous silica films have applications in catalysis, environmentalremediation, energy storage, thermal barriers and energy storage. Poroussilica films, in particular, are potentially useful as low dielectricconstant interlayers in semiconductor devices, as low dielectricconstant coatings on fibers and other structures, and in structuredcatalytic supports. Porous silica films produced by previous methods canbe divided between random, gel-like silica films, andsurfactant-templated films in which the pores are within a hexagonallattice, with the characteristic pore diameter defined by the surfactantmicelle.

Previous work resulting in mesoporous membranes fromsurfactant-templated powders and structures by in-situ solution-phaseprecipitation has been described in co-pending U.S. patent applicationSer. No. 08/344,330. In-situ solution-phase precipitation requiressubstantial time from about 4 hours to 1 week to form a mesoporousmembrane or film.

Hrubesh, L. W.; Poco, J. F., J. of Non-Cryst. Solids 1995, vol. 188, p.46 applied “aerogel” technology to produce high-porosity films withrandom porosity. In the aerogel synthesis route, a hydrolyzedsilicon-alkoxide solution is metered onto a spinning substrate. To avoiddrying, the spinning apparatus is in an atmosphere saturated withsolvent vapor. The spinner is stopped with a brake, and the retainedspinning solution gels within a few minutes. The gel-coated substrate isimmersed in solvent and subsequently dried under supercriticalconditions.

Smith et al. (Smith, D. M.; Anderson, J.; Cho, C. C.; Gnade, B. E., Mat.Res. Soc. Symp. Proc. 1995, 371, 261, and Smith, D. M.; Anderson, J.;Cho, C. C.; Johnston, G. P.; Jeng, S. P., Mat. Res. Soc. Symp. Proc.1995, 381, 261) applied “xerogel” technology as an alternative toaerogels. Here, the spin-cast silica sol-gel film is aged, washed andsolvent exchanged, silated with a trimethylchlorosilane solution inheptane, and dried. In contrast to the aerogel process, the film isdried at ambient pressure. The aging and chemical treatment minimizespore shrinkage during drying and makes the film hydrophobic, but thefilm becomes hydrophilic on heat-treatment, unless done in a forming gasenvironment.

Both techniques for spin-casting (1) aerogel and (2) xerogel films arecomplicated by the fact that spinning must be performed insolvent-saturated atmospheres (requiring explosion proofing) to avoidpremature drying of the film.

In other work on mesoporous silica films, Ogawa (Ogawa, M., J. Am. Chem.Soc. 1994, 116, 7941) fabricated spin-cast silica-CTAB films. Ogawa useda CTAB/TMOS mole ratio of 0.40 in a solution that avoided gelation orprecipitation and produced films that were lamellar, containingalternating layers of silica and bilayers of CTAB, and therefore notcalcinable; surfactant can not be removed without degradation of thefilm structure. Accordingly, Ogawa did not calcine his silica films.Although Ogawa noted that rapid evaporation was essential for theformation of highly-ordered, lamellar CTAB-silica composites, thosecomposites would not be expected to be stable to calcination, and wouldalso not contain useful pore structures.

Further work by Ogawa (M. Ogawa, A SIMPLE SOL-GEL ROUTE FOR THEPREPARATION OF SILICA-SURFACTANT MESOSTRUCTURED MATERIALS, Chem.Commun., 1996, 1149-1150) was with a CTAC/TMOS ratio of 0.25. However,he used a substoichiometric ratio of water to silica (TMOS) of 2(stoichiometric ratio of water to silica is 4). Ogawa's product, beforecalcination, has the 100, 110 and 200 reflections in the XRD patterncorresponding to a hexagonal structure. However, no information is givenon calcined films in which the surfactant has been removed. It isinferred that Ogawa's product is unstable against calcination.

Porous silica fibers, with ordered porosity in the nanometer scale, havepotential applications in catalysis, environmental remediation, thermalinsulation and chemical sensors. Nanoporous or mesoporous fibers usingthe previously described sol-gel methods and stable against calcinationhave not been reported.

In the previous methods in the literature, there is no direct means forcontrolling particle size or pore volume fraction in powder, films orfibers.

Accordingly, there remains a need for mesoporous products having welldefined morphology on both the nanometer scale (1-20 nm) (solid silicaand pores) and the micrometer scale (0.1 μm-100 μm) (the characteristicdimension of the mesoporous product), and a method for making them inless time and without the need for filtration. Where spin-casting isdone, there remains a need for a straight-forward method for producingmesoporous film(s) without supercritical drying, aging, silation of thefilm(s), or controlled gas environments.

SUMMARY OF THE INVENTION

This invention pertains to the development of surfactant-templated,nanometer-scale porosity of a silica precursor solution and forming amesoporous material by first forming the silica precursor solution intoa preform having a high surface area to volume ratio, then rapid dryingor evaporating a solvent from the silica precursor solution. Themesoporous material may be in any geometric form, but is preferably inthe form of a film, fiber, powder or combinations thereof. The rapiddrying or evaporation of solvent from the solution is accomplished byforming a preform by any of layer thinning, for example spin casting;drawing; or spraying respectively.

It is critical to the present invention that the silica precursorsolution avoid gelation or precipitation in order to permit formation ofthe mesoporous material by templating and evaporation of solvent(s). Ina precipitation process, the composition of the mesoporous material isgoverned by a partitioning between the aqueous silica precursor solutionand solid phases. In the evaporative process of the present invention,mesoporous material composition is directly related to the compositionof the silica precursor because all of the non-volatile components ofthe precursor solution (namely the silica and the surfactant) areincorporated into the mesoporous structure. Therefore, in the presentinvention, the pore volume per gram of silica is controllable by varyingthe surfactant to silica precursor mole ratio in the silica precursorsolution.

It is further critical to the present invention that the silicaprecursor be an alkoxide silica precursor, and that the mole ratio of anamount of a surfactant to the alkoxide silica precursor for templatingbe great enough to avoid producing a dense, non-porous film yet lowenough to avoid producing a lamellar structure that is not calcineable,or any other non-calcineable structure. Left to themselves, somealkoxide silica precursor solutions will gel or precipitate over time ifleft alone from about 10 seconds to about 5 days or a week dependingupon the solution. Thus, preforming must be done within a time beforegelation or precipitation occurs. Finally, the rate of evaporation iscritical to the formation of the mesoporous product. The slower theevaporation, the less ordered the mesopores. Accordingly, it ispreferred that the solvent be evaporated or removed from the templatedmesoporous structure in a time less than about 5 minutes, preferablyless than about 1 minute, and most preferably less than about 10seconds.

Production of a film is by layer thinning, wherein a layer of the silicaprecursor solution is formed on a surface followed by removal of anamount of the silica precursor solution and leaving a geometricallythinner layer of the silica precursor solution from which the solventquickly escapes via evaporation. Layer thinning may be by any methodincluding but not limited to squeegeeing and/or spin casting. Spin-castmesoporous films are formed on the order of a minute or even seconds.

Advantages for the layer thinning method of the present inventioninclude (1) films are formed within a minute (apart from time requiredfor post-treatment and calcination), (2) no special atmospheres,pressures or supercritical drying equipment are required as in the caseof aerogel film fabrication, and (3) the porosity is ordered, and of acontrolled pore size rather than a random, gel-like structure in thecase of aerogels and xerogels; the volume fraction of porosity and thestructural order within the film are controllable by the silica tosurfactant content or mole ratio. Further advantages are realized fromthinning with a spin-coater, which is standard equipment in themicroelectronics industry. Advantages of using a spin-coater include (1)films have uniform interference colors, indicating uniform filmthicknesses (2) film thicknesses are repeatable from sample to sample(for example ±0.006 μm thick film, or a 1% variation) and controllableby varying the ethanol and water dilution and the spinning speed, and(3) the spin-casting technique does not require the use of largesolution batches in which only a small fraction of the solution is usedfor film growth, as in the case of the earlier film growth technique forwhich the solution is depleted with film growth and must either bereplaced or somehow regenerated. In the interfacial growth technique,bulk solutions of silicate and surfactant are used.

In powder formation by spray drying, the same conditions of fast dryingexists as in spin-casting (as well as in fiber spinning) because of thehigh surface-area to volume ratio of the product. When a powder isproduced by liquid spraying, the particles or micro-bubbles within thepowder are hollow spheres with walls composed of mesoporous silica.

The volume within the interior of the mesoporous micro-bubble isundesirable for some applications including catalytic processes in whichcoking occurs. In these cases, the micro-bubbles may be broken bycrushing or grinding. Mesoporous silica powders impregnated withcatalytically active metals have applications in catalysis. The poresize, ˜25-40 Å, allows access of large molecules to catalysis sites. Thehigh surface area of the powders allows high catalytic activity. Thesurface area of mesoporous powders was determined to be ˜900 m²/g bynitrogen absorption. The powders may be pressed or mixed with bindersand extruded to produce pellets, tubes and other shapes for structuredcatalyst supports. Thus, the particle size in spray-drying may becontrolled for a particular application. Because the micro-bubble wallsare permeable, many applications such as micro-encapsulation ispossible. Silica is ingestible. Containing a drug within themicro-bubble to allows passage through the stomach where it wouldnormally degrade. The drug is released through the porous walls into theintestinal tract. Materials, including surfactants and polymers,adsorbed to either the outside of the bubble or within the pores canacts as pH-sensitive gates for the release of the drug.

Encapsulation may be done wherein a non-drug substance may bepermanently caged within the bubble by closing off the pores with silanetreatment, silica precipitation, or surfactant absorption.

Mesoporous fiber formation starts with a similar silica precursorsolution but with an added pre-polymer making a pituitous mixture. Thepituitous mixture is drawn into a thin strand from which solvent isevaporated leaving the mesoporous fiber(s). Mesoporous silica fibers maybe impregnated with catalytically active metals for applications instructured catalytic packing. The small thickness, on the order of 10 to100 μm, minimizes the diffusion distance from the bulk to the catalyticsites on the internal surface of the silica. The high aspect ratio ofthe fibers give the advantages of high throughput, combined with highreactive areas. The fibers may be wound or assembled in reactor modules.Hollow mesoporous fibers fabricated by rapid drying with heated gas(e.g. air) may be bundled into a module to form a catalytic membranereactor. Reactants can flow through the hollow fiber and diffuseradially outward, through the mesoporous wall, past catalytically activesites. The reactor is especially useful for reactions where shortcontact times and good temperature control are required (e.g. partialoxidation). The temperature is uniform because reactions occur along thelength of hollow fibers. The high surface area of the fibers allows highcatalytic activity. The surface area of mesoporous fibers was determinedto be ˜1100 m²/g by nitrogen absorption. Mesoporous fibers have furtheruse in high-performance thermal insulation. The pore size, ˜25-40 Å, issuch that transport of phonons of specific wavelength is reduced,limiting conductive heat transfer within the fiber.

It is an object of the present invention to provide a method of makingmesoporous materials having high surface area to volume ratios.

It is a further object of the present invention to provide a method ofmaking the mesoporous materials by solvent evaporation.

The subject matter of the present invention is particularly pointed outand distinctly claimed in the concluding portion of this specification.However, both the organization and method of operation, together withfurther advantages and objects thereof, may best be understood byreference to the following description taken in connection withaccompanying drawings wherein like reference characters refer to likeelements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows XRD patterns for precalcined (A) and post calcined (B)mesoporous silica film.

FIG. 2 shows the d-spacing for the precalcined (solid diamonds) and postcalcined (open squares) mesoporous silica films.

FIG. 3 shows the XRD primary reflection peak height for the precalcined(solid diamonds) and post calcined (open squares) mesoporous silicafilms.

FIG. 4 shows the volume fraction (open circles) and index of refraction(solid squares) for the calcined mesoporous silica films.

FIG. 5 shows (A) the PXRD pattern for the evaporated silica precursorsolution and (B) the XRD pattern for the mesoporous silica film.

FIG. 6 is a XDS of the mesoporous silica powder particle.

FIG. 7 shows PXRD patterns for precalcined and post calcined mesoporoussilica powder.

FIG. 8 is a PXRD pattern for the precalcined mesoporous silica fiber.

FIG. 9 shows PXRD patterns for precalcined (A) and post calcined (B)mesoporous silica fiber.

FIG. 10 shows PXRD patterns of mesoporous silica fibers where trace A isair-dried fibers and trace B is calcined fibers.

FIG. 11 shows nitrogen adsorption/desorption curves for the mesoporousfibers.

FIG. 12 shows pore-size distribution of the mesoporous fibers.

FIG. 13 shows pore volume fraction and the surface area of calcinedspray-dried powders as a function of the surfactant to silica moleratio.

FIG. 14 shows nitrogen adsorption/desorption curves for the mesoporouspowder with CTAC/TEOS ratio of 0.28.

FIG. 15 shows pore-size distribution of the mesoporous powder withCTAC/TEOS ratio of 0.28.

FIG. 16 shows PXRD patterns of calcined spray-dried powders fordifferent surfactant to silica mole ratios.

FIG. 17a shows Al-NMR data of mesoporous aluminosilicates, Al:Si moleratio of 0.25 prior to calcination.

FIG. 17b shows Al-NMR data of mesoporous aluminosilicates, Al:Si moleratio of 0.031 after calcination.

FIG. 17c shows Al-NMR data of mesoporous aluminosilicates, Al:Si moleratio of 0.063 prior to calcination.

FIG. 17d shows Al-NMR data of mesoporous aluminosilicates, Al:Si moleratio of 0.063 after calcination.

FIG. 18 shows PXRD patterns of as-synthesized spray-dried powders fordifferent aluminum to silica mole ratios.

FIG. 19 shows PXRD patterns of calcined spray-dried powders fordifferent aluminum to silica mole ratios.

FIG. 20 shows XRD patterns for precalcined (A) and post calcined (B)mesoporous silica film spun using precursor solution #56483-2.

FIG. 21 shows XRD patterns for precalcined (A) and post calcined (B)mesoporous silica film spun using precursor solution #56483-5.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The method of the present invention relies upon a silica precursor mixedwith a surfactant in an aqueous solution for templating the silicaprecursor together with a catalyst (acid) for hydrolysis of the silicaprecursor. The silica precursor is then made into a preform that has ahigh surface area to volume ratio and the aqueous solution quicklyevaporated to form the mesoporous material. The evaporative process formesoporous material has the following steps: (1) the silica precursor inaqueous solvent is formed into a preform of high surface area to volumeratio (by spinning, drawing, atomizing) and (2) the solvent is quicklyevaporated, leaving mesoporous silica in a similar shape (film, fiber,sphere). High surface area is necessary for fast evaporation of thesolvent. A third step of heating may be used to remove any residualsolvent and to further condense the silica, followed by calcining whichfurther removes any residual surfactant.

The evaporation is fast in comparison to precipitation. With the preformhaving a high surface area to volume ratio, and with heated air, solventis rapidly evaporated from the preform. Mesoporous materials are formedin less than five minutes, preferably less than one minute. Forparticles, a particle may be formed in less than one second (<<1 s). Ina few minutes of spray drying, several grams of powder are produced. Drypowders are formed directly in the spray-dryer and no filtration step isrequired.

In spray drying, the precursor solution is atomized into fine droplets.Solvent evaporation leaves behind a shell of mesoporous silica. Inspraying, the droplet size, and thus the mesoporous particle size, iscontrolled by modifying the rheological properties of the sprayingsolution (through solvent dilution or addition of polymeric thickeners)and by changing spray conditions (by the use of different nozzlegeometries and varying solution pressure).

The silica precursor may be an alkoxide silica precursor ortetrachlorosilane. A preferred alkoxide silica precursor is tetraethylortho-silicate (TEOS). Other alkoxide silica precursors includeorthosilicates, including but not limited to tetramethyl orthosilicate(TMOS), tetrapropyl orthosilicate, and tetrabutyl orthosilicate.Iso-propyl, sec-butyl and tert-butyl orthosilicates are included as wellbut may have limited commercial availability. In addition to thealkoxide, a metal halide salt may be added, especially a metal chlorideas well as a metal nitrate. Metal halide salt(s) and/or metal nitrate(s)combine with the alkoxide. More specifically, iron chloride or nitrate,aluminum chloride or nitrate combines with the alkoxide. Additionalmetal(s) may be incorporated into the mesoporous silica structure. Theseadditional metal (s) result in reducing solubility of the mesoporoussilica structure and may impart a negative charge to the mesoporoussilica structure.

A preferred surfactant contains an ammonium cation, either a quaternaryammonium cation, for example cetyltrimethylammonium chloride (CTAC), ora tertiary ammonium cation. Variations of CTAC as described by Huo (SeeBackground) include substitution of ethyl and propyl groups for themethyl group that may also be used. In addition, it is possible toproduce mesoporous materials using alkyl trimethylammonium chloride orbromide surfactants with different alkyl chain lengths. Variation inalkyl chain length (e.g. C₁₂, C₁₄, C₁₆, C₁₈) permits control of the porediameter wherein shorter alkyl chain lengths produce smaller diameterpores.

An alternative method of varying pore size is by adding a swelling agentto the silica precursor solution. For example, addition of1,3,5-trimethylbenzene produces pore diameters about 2-5 times greaterthan pores made without the swelling agent.

In the present invention, the mole ratio of templating surfactant (orsurfactant) to alkoxide silica precursor is preferably from about 0.05to about 0.3. Below about 0.05, a dense, non-porous silica phase isproduced and above about 0.3, a lamellar phase is produced that is notcalcineable. The lamellar phase is not calcineable because the lamellarstructure collapses upon removal of the surfactant that occurs duringcalcination. Specifically for CTAC/TEOS, the mole ratio is preferablyless than about 0.24. More specifically, with a mole ratio of betweenabout 0.1 to about 0.15, the pores are well ordered. However, at highratios, above 0.24, specifically from about 0.26 to 0.28, give high porevolume fraction and high specific surface area (e.g. 1770 m²/g),surprisingly higher than reported in the prior art.

In the present invention, the mole ratio of water to silica precursor ispreferably greater than or equal to a stoichiometric ratio. Morepreferably the ratio of water to silica precursor is about 7. Use ofstoichiometric or super stoichiometric amounts of water is believed tohelp preserve the hexagonal structure of the product upon calcination.

Control of the morphology on the micrometer scale is also unique to theevaporative process. In spin casting, a flat substrate flooded with theprecursor solution is accelerated to high rpm. Excess solution flows offduring spinning, leaving a thin film of the solution which forms a solidmesoporous film by evaporative concentration. Films may be deposited onno-flat surfaces by spraying, painting or dip coating. It should benoted that according to the present invention it may be advantageous toinsure that the substrate surface is hydrophilic. For silicon wafersubstrate, a hydrophilic surface may be obtained by sonication indeionized (d.i.) water, followed by soaking in a solution of sulfuricacid and finally rinsing with d.i. water and drying.

In fiber spinning, the precursor solution (mixed with highmolecular-weight polymer) is drawn into a strand. The solutionevaporates leaving the mesoporous fiber. Drawing may be either bycontacting an object to the precursor solution and moving the objectaway and forming a strand of precursor solution, or by permitting theprecursor solution to flow from a vessel under pressure or by gravity.Flow under pressure may include extrusion.

In the hydrolysis of the silica precursor, tetrahydroxysilane isproduced which undergoes condensation reactions to form silicaoligomers. With the alkoxide silica precursor, an alcohol is a byproductof hydrolysis. With the tetrachlorosilane precursor, hydrochloric acidis a byproduct.

EXAMPLE 1

An experiment was conducted to demonstrate making mesoporous films bythe method of the present invention.

Silicon wafers were obtained from Silicon Source and cut into 2.5×2.5cm² squares. The silicon wafers were pretreated by sonication indeionized (d.i.) water, followed by soaking overnight in a solution ofsulfuric acid and Nochromix™ (Godax Labs) and finally rinsing with d.i.water and drying by aspiration.

The silica precursor solution had the mole ratios of TEOS (Aldrich) 1.0;deionized water 7.2 (18 MΩ resistance); ethanol (punctilious; QuantumChemicals) 5.7; HCl (Mallinckrodt) 0.10. The (CTAC) (T.C.I. America) wasadded after hydrolysis. CTAC/TEOS mole ratio was varied from 0 to 0.30to determine its influence on film properties.

Spin-castings were performed with a Specialty Coating System ModelP-6204A spin coater. With the silicon wafer at rest, the entire topsurface of the silicon wafer was covered with the hydrolyzedTEOS-surfactant solution. The covered silicon wafer was spun at 4000 rpmfor 60s with maximum acceleration (spin-up time <1s). A substantialamount of the hydrolyzed TEOS-surfactant solution flowed off the coveredsilicon wafer during rotation. The remaining solution was geometricallyat a high surface area to volume ratio. Flowing ceases as the viscosityof the hydrolyzed TEOS-surfactant solution increases because of theincreased concentration of silica precursor and surfactant from loss ofsolvent through evaporation.

The precalcined mesoporous silica film was post treated with ammoniavapors which improved the stability of the pore structure aftercalcination. A few drops of ammonium hydroxide were put in a closedpetri dish containing the spin coated silicon wafer. The spin coatedwafer was ammonia treated for about 15 minutes.

The spin coated silicon wafer was removed from the ammonia treatmentthen heated to 105° C. for several hours to complete drying of thesolvent and increase condensation of the silica. Finally, the dried spincoated silicon wafer was calcined at 550° C. for 5-10 minutes.

The mesoporous silica film was characterized by X-ray diffraction andellipsometry. FIG. 1 shows XRD patterns of the film before calcination Aand after calcination B for a CTAC/TEOS mole ratio of 0.12. The (100)and (200) reflections are apparent. The absence of the (110) reflectionsuggests orientation of the c-axis parallel to the substrate (siliconwafer) surface. The increase in the peak heights of the reflectionsafter calcination B was believed to result from the increaseddifferences in scattering density between the silica walls and the poresafter the surfactant was burnt out. The peak width (the full width athalf maximum) of the (100) reflection was nearly constant withcalcination, indicating the stability of the mesoporous structure. Thepeak height was also a qualitative indicator of a well-ordered filmstructure. The d-spacing of the first diffraction line for the hexagonalstructure is 33 Å after calcination. Consolidation of the silica withcalcination caused a 3 Å contraction in the (100) reflection. Calcinedmesoporous silica films having d-spacing less than 40 Å have not beenachieved prior to the present invention (see FIG. 2). Preferably thed-spacing is less than 39 Å, more preferably less than 38 Å, and mostpreferably less than 37 Å.

FIG. 2 shows the d-spacing of the primary X-ray reflection, before andafter calcination, as a function of the CTAC/TEOS mole ratio. Noreflections exist for the film without surfactant. With an increasingmole ratio up to 0.15, the d-spacing of the un-calcined filmmonotonically decreases down to 36 Å. Above a ratio of 0.15, thed-spacing abruptly increases to 39-41 Å and is roughly constant athigher surfactant concentrations. The peak height of the primaryreflection, which is a qualitative indicator of structural ordered, isshown in FIG. 3. In the mole ratio range between 0.1 and 0.15 a maximumexisted, demonstrating that order was improved by optimizing theCTAC/TEOS mole ratio. The pores were in a well-ordered hexagonallattice. In the mole ratio range of >0 to 0.21, the porosity wasthermally stable; the peak height either increased or remained constantafter calcination. For ratios ˜0.24 and above, the peak height for theun-calcined film was large. However, the films had a cloudy appearanceand would not be suitable for many applications. The drop in the peakheight and the increase in the peak width (not shown) after calcinationindicate poor thermal stability. Therefore, the desired CTAC/TEOS moleratio range is less than 0.24.

Although the pores were hexagonally ordered within a narrow compositionrange, within a broader range the films were thermally stable and theporosity may be fine-tuned by adjusting the CTAC/TEOS ratio. FIG. 4shows the volume of silica, determined by ellipsometry, of calcinedfilms as a function of the CTAC/TEOS mole ratio. The solid curve is theexpected volume fraction based on the volume contributions of the silicaand the surfactant and the volume shrinkage indicated by shifts in thepositions of the X-ray reflections after calcination. The correspondenceof the curve with the data demonstrates that the same CTAC/silica moleratio existed in the film as in the spin-casting solution. Pore volumesup to ˜64 vol % (˜36% silica) were measured in films prepared with aCTAC/TEOS ratio of 0.20.

FIG. 4 also shows the index of refraction at a 500 nm wavelength ofcalcined films as a function of the CTAC/TEOS mole ratio. The index ofrefraction gives an indication of the dielectric constant because thesquare of the index of refraction is the dielectric constant at highfrequencies. The data shows the index of refraction (and the dielectricconstant) is tunable by varying the CTAC/TEOS mole ratio. The index ofrefraction values ranging from that of silica down to 1.16. Ellipsometrywas performed immediately following heat treatment at 450° C. For themost porous sample, the index of refraction increased less than 1% overone week in ambient air.

Calcined films were characterized by X-ray photoelectron spectroscopywith analyzed volume on the surface of approximately 1×1 mm² in area and20 to 40 Å in depth. Silicon, oxygen and a small amount of adventitiouscarbon were identified. Within the resolution of the equipment, nochlorine nor nitrogen were found, demonstrating that calcination yieldsrelatively pure silica without contamination from other chemicals usedin the process. Cross-polarized optical microscopy of films deposited onglass slides, before and after calcination, did not reveal liquidcrystalline-like optical anisotropy. Atomic force microscopy of themesoporous film (not shown for brevity) revealed a surface morphologydominated by undulations ˜1 μm in diameter and raised rims ˜0.1 μm inwidth. Height variations were within 60 nm. The film was continuous andnot the result of the deposition of pre-existing particles.

EXAMPLE 2

An experiment was conducted to demonstrate that quick drying wasimportant to the film structure.

Silicon wafers were pre-treated in the manner described in Example 1.

The silica precursor solution had mole ratios of deionized water 7.1;ethanol 5.4; HCl 0.1; TEOS 1.0; and CTAC 0.11.

Spin castings were performed in the manner described in Example 1. Theremaining silica precursor solution was evaporated by natural convectionin an open glass bottle. The spin coated wafer and the evaporated silicaprecursor solution were not post treated with ammonia vapors. The spincoated silicon wafer and evaporated silica precursor solution wereheated to 105° C. for several hours.

The mesoporous silica film and the evaporated silica precursor solutionwere characterized by XRD. FIG. 5 shows the XRD pattern for themesoporous material film and the PXRD pattern for the evaporated silicaprecursor solution. The mesoporous silica film has a strong primaryreflection, a qualitative indicator of structural order. The evaporatedsilica precursor solution has only a broad peak of very low intensityand thus exhibits poor pore ordering. These results demonstrate thatsilica precursor solutions evaporated by natural convection do not yieldordered mesoporous silica.

EXAMPLE 3

An experiment was conducted to demonstrate making mesoporous materialpowder. The silica precursor solution was prepared with the followingcomposition by mass: TEOS 51.80 g; water 26.264 g; 38 wt % hydrochloricacid 1.756 g; and CTAC 10.333 g. TEOS, deionized water and HCl werefirst combined together, following by addition of CTAC. Normallyimmiscible, TEOS combines with water in the presence of the surfactant,allowing the hydrolysis reaction to occur. The solution became hot uponmixing from the exothermic hydrolysis; the sample bottle was cooledunder running water.

In Example 1, ethanol was used to dilute the precursor solution for spincasting. However, in the spray-drying process, potential explosion orflammability hazards from either added flammable solvents or the ethanolreaction by-product must be minimized. Hence, no ethanol dilution wasused because of the potential hazards. Instead of ethanol, water wasused for dilution, hydrolysis, and solvent for the surfactant.

The precursor solution was spray-dried in a Büchi-190 Mini Spray Dryeroperating with an inlet air temperature of 174° C., an outlettemperature of 76° C., a pump speed setting of “5,” a heat setting of“13,” an airflow setting of 300 and a gate valve setting between ¼ and½. The precursor solution was pumped through a water-cooled nozzle intoa flow of heated air and down the length of a ˜30 cm drying tube. Thesolvent in the droplets of the spray quickly evaporate, leaving behindthe nonvolatile material in the particulates which are collected at thebottom of a venturi separator. It takes on the order of a second formaterial to pass through the spray nozzle and be collected.

The powder was heated at 105° C. for ˜2 h, followed by calcination at600° C. for ˜30 min. The absence of ammonia treatment for this powderdemonstrates that ammonia is not required for producing a calcineablematerial. After calcination, the powder lost 40.6% of its mass, comparedwith a theoretical mass loss of 40.9% based on the precursor solutioncomposition (assuming silicon exists in the form of SiO₂ and eachsurfactant molecule has a chloride counter-ion). The close agreementbetween the experimental and theoretical values supports the assertionthat, in the rapid drying process, the mesoporous solid composition isdirectly determined by the solution composition, and all of thenonvolatile species are incorporated into the solid.

Scanning electron micrographs of the spray-dried mesoporous silica aftercalcination showed that the particles were in the form of hollow spheresor shells with diameters ranging from approximately 4 to 40 μm (micron).Potentially, hollow spheres may range in size from about 1 μm to about300 μm. The wide range of particle size was probably due to non-uniformatomization from the spray nozzle. The hollow nature of the particleswas evident by the fraction of collapsed particles which have theappearance of deflated balls. From the width of the folds in thecollapsed particles, the thickness of the particle bubble shell wasestimated to be less than 0.5 μm. Electron dispersive X-ray analysisconfirmed the calcined particle micro-bubbles were composed only ofsilicon and oxygen (see FIG. 6).

A surfactant-silica solution has been spray dried into a powder. X-raydiffraction data of calcined powder clearly showed the existence ofmesoporosity by a low-angle primary peak corresponding to a d-spacing of˜31 Å.

FIG. 7 shows powder X-ray diffraction (PXRD) patterns in the low-anglerange of the powder sample prior to and after calcination. The primarypeak, corresponding to a d-spacing of 34 Å, prior to calcinationindicates the average spacing between reflection planes. Aftercalcination, the primary peak corresponds to a d-spacing of 31 Å due toshrinkage. The existence of the primary peak after calcinationdemonstrates that the pores are stable with calcination. The intensityof the reflections was higher after calcination which is probably aresult, in part, of the increase in the scattering density contrastafter surfactant burnout.

In the PXRD pattern of the powder after calcination, a second peak oflow intensity at 5.1°, corresponding to a d-spacing of 17.3 Å,corresponding to a d-spacing of 17.3 A, is apparent. The (110) and the(200) reflections were not clearly resolved.

EXAMPLE 4

An experiment was conducted to demonstrate mesoporous silica fibers.Poly(ethyleneoxide) (PEO) with a MW of 5×10⁶ was mixed with 18 MΩdeionized water to form a 3.7 wt % stock solution and was allowed todissolve overnight. The pituitous mixture was prepared with thefollowing composition by mass: TEOS 8.02 g; water 3.60 g; 38 wt %hydrochloric acid 0.39 g; CTAC 1.71 g, and 3.7 wt % PEO solution 1.26 g.TEOS, deionized water and HCl were first combined together, followed byCTAC. The solution became warm upon mixing from the exothermichydrolysis; the sample bottle was cooled under running water. The PEOsolution was then added to modify the rheology of the silica/CTACsolution to allow drawing of fibers.

Fibers were drawn onto a spindle with four wooden dowels. The dowelswere covered with parafilm™, on which fiber samples were collected. Thespindle was driven either by hand or by an electric hand drill attachedby the drill chuck to the end of the metal rod. A thin, stream ofsolution, was drawn up from the solution with a pipet tip, and woundonto the spindle. The rotating spindle served to collect, pull andrapidly dry the fibers. After drawing, the fibers were peeled away fromthe dowels and collected in a crucible.

The fibers were ammonia treated for ˜1h by dripping a few drops ofammonium hydroxide solution onto a tissue taped to underside of a coverand placing the cover over the crucible containing the fiber sample. Theammonia vapor raises the pH in the fibers, and increases condensation ofthe silica and improves the stability of the porous silica structureduring subsequent high-temperature calcination. The fibers were placedin an oven at 105 to 180° C. for several hours, followed by heattreatment at 600° C. for ˜1 h. The fully calcined fibers were white,presumably due to opacity arising from large defects incorporated duringthe hand-drawing procedure. The mass loss after calcination was 55%compared with an expected value of 57% calculated based on the amount ofSiO₂, CTAC, and polymer that would be obtained from the pituitousmixture.

The diameter of the fibers varied from 5 to 100 μm with a range of 35 to60 μm being more typical. Fibers with small diameter of ˜10-30 μm aredesirable because of better bending endurance and weavability. Smalldiameters are easily achieved with state-of-the-art fiber drawing(extrusion) equipment.

Fibers were ground with a mortar and pestle and characterized by powderX-ray diffraction (PXRD). FIG. 8 shows the PXRD pattern of fibers priorto calcination along with the pattern for the as received PEO. The broadamorphous peak centered at a 2θ value of 21° corresponds to silica. Asreceived PEO is crystalline. However, no crystalline peaks are observed,indicating the polymer is dispersed within the fiber material.

FIG. 9 shows the PXRD patterns in the low angle range of the fibersample prior and after calcination. The primary peak at 2.3°,corresponding to a d-spacing of 38 Å, prior to calcination indicates theaverage spacing between reflection planes. After calcination the primarypeak shifts to 2.8° (32 Å) due to shrinkage. The existence of theprimary peak after calcination demonstrates the pores are stable withcalcination. The intensity of the reflections is higher aftercalcination which is probably a result of the increase in the scatteringdensity contrast after surfactant and polymer burnout. A similarintensity increase after calcination was observed for the mesoporousfilms described in Example 1.

A second peak at low intensity at 3.9°, corresponding to a d-spacing of23 Å, (4.4° or 20 Å after calcination). The (110) and (200) reflectionsare not clearly resolved similar to the mesoporous powder XRD patternfrom example 3. The pore ordering in the fibers is improved by varyingthe CTAC/TEOS mole ratio and by drawing smaller diameter fibers.

TEM photos confirm a well ordered mesoporous structure within thefibers. Cross-polarized optical microscopy of the calcined fibersrevealed liquid crystalline-like optical anisotropy.

EXAMPLE 5

An experiment was performed to demonstrate incorporation of aluminuminto the mesoporous silica film.

Silicon wafers were pre-treated in the manner described in Example 1.

The aluminosilicate precursor solutions had mole ratios of deionizedwater 7.3; ethanol 5.3; HCl 0.09; CTAC 0.14; TEOS 1.0. The mole ratio ofaluminum nitrate (Al(NO3)3 9H2O) to TEOS mole ratio was varied between0.035 to 0.25. The solutions were prepared by combining deionized water,ethanol, hydrochloric acid, CTAC and aluminum nitrate together, followedby the addition of TEOS.

Spin castings were performed in the manner described in Example 1.

The spin coated wafers were not post treated with ammonia vapors.

The spin coated wafers were heated and calcined in the manner describedin Example 1.

The calcined mesoporous aluminosilicate films were characterized by XRDin a 2-theta range of 1-10°. XRD results for the primary reflections aresummarized in Table E6-1. With increasing Al/TEOS mole ratio, thed-spacing of the primary reflection decreased. The calcined mesoporousaluminosilicate film with a Al/TEOS mole ratio of 0.064 had the greatestprimary reflection peak intensity. However, an Al/TEOS mole ratio of0.064 should not be considered an optimum value for a well-ordered filmstructure because the pore ordering is also dependent on the surfactantcontent in the aluminosilicate precursor solutions. The dependence ofthe pore ordering on the CTAC/TEOS mole ratio was demonstrated inExample 1 for mesoporous silica films without aluminum. In the XRDpatterns of the calcined mesoporous aluminosilicate films with Al/TEOSmole ratios of 0.035, 0.064 and 0.13, reflections of low intensity wereobserved at half the d-spacing of the primary reflection.

TABLE E6-1 XRD results for the primary reflections Al/TEOS mole ratiod-Spacing/Å Peak Intensity/cps 0.035 35 2800 0.064 30 9700 0.13 27 64000.25 25 1700

To identify possible crystalline phases distinct from the amorphous porewalls, the calcined mesoporous aluminosilicate film with an Al/TEOS moleratio of 0.25 was characterized by XRD in a 2-theta range of 5-30° usinga slow scan rate (0.04°/75 s). A wider 2-theta range was not usedbecause of the strong reflections from the silicon wafer substrate above30°. A low intensity peak with a d-spacing of 4.02 Å was observed. Theonly possible matching reference data for the Si—Al—O—H system was forcristobolite (SiO2). Therefore, the XRD pattern did not show a separatealuminum-oxide crystalline phase.

The calcined mesoporous aluminosilicate film with a TEOS/Al mole ratioof 0.25 was characterized by SEM. The calcined mesoporousaluminosilicate film was homogeneous; no crystal gains were observed. Asmall amount of surface roughness was observed which had the sameappearance of the AFM image discussed in Example 1. EDS characterizationof the calcined mesoporous aluminosilicate film showed the presents ofaluminum. The EDS characterization was not quantitative because ofsignificant penetration of the electron beam through the calcinedmesoporous aluminosilicate film and into the silicon wafer substrate.

EXAMPLE 6

Dry-Spun Mesoporous Fibers

An experiment was conducted to demonstrate making well-orderedmesoporous fibers by the method of the present invention.

The spinning solution was formed by combining deionized water,hydrochloric acid (Mallinckrodt), 5×10⁶ MW poly(ethylene oxide) (PEO)(Polysciences) from a 4 wt % aqueous stock solution, ethanol(punctilious, Quantum Chemicals), followed by TEOS (Aldrich). Thesolution was mixed to promote the hydrolysis reaction. Finally, CTAC wasadded to obtain final mole ratios of: 7.0 H₂O, 0.050 HCl, 0.10 PEO(repeat unit), 4.0 ethanol, 1.0 TEOS, 0.24 CTAC. A thin strand of thepituitous solution was drawn from a pipette tip, and wound at a rate of300 m/min onto a spool consisting of six dowels. Fibers were air driedat 105° C. overnight, and calcined by heating at 350° C. for 1 h and600° C. for 3 h.

Samples were analyzed by powder X-ray diffraction using a Philipsdiffractometer with Cu Ka radiation. Pore-size distributions and BETsurface areas were determined from nitrogen adsorption/desorptionisotherms with the Quantachrome Autosorb 6-B gas sorption system, usingthe BJH and multi-point BET methods, respectively.

Mesoporous fibers were dry spun by drawing the precursor solution intocontinuous filaments and collecting on a spool. Fibers crossing on thespool during spinning tend to fuse together, creating a gauze-likeproduct at the end of spinning which was cut away in sections frombetween the dowels. The as-spun fibers were pliable and pressable intopellets or rolled into tubes. With drying and calcination the fibersbecome brittle. Low temperature oven drying promotes condensationbetween silica oligomers and increases calcination stability of thesilica phase. During calcination, silica undergoes further condensation;surfactant and polymer are removed, leaving the porous structure. Themass loss of 59% after calcination compared to a value of 57% calculatedfrom the spinning solution composition, assuming the dried fiberscontain SiO₂, PEO, and surfactant with chloride bridging-ions (for thepowders, mass losses were within 1% of the calculated values). Theexcess loss was attributed to incomplete drying/silica condensationprior to calcination.

A scanning electron micrograph of these calcined fibers showed fiberdiameters are on the order of 40 μm and where varied by modifyingsolution composition and spinning conditions. The distribution of fiberdiameters was due to the hand spinning technique presently used; moreuniform fibers are achievable with state-of-the-art spinning equipment.The fiber cross sections typically had a kidney-shape, characteristic ofdry-spun fibers where high evaporation rate at the air-fiber interfaceand comparatively slow solvent diffusion rates through the fiber causedthe skin to collapse around the soft cores. Self-assembly of silica andsurfactant occurred first at the air-fiber interface, followed byprogressive conversion of the entire fiber to a mesophase structure. Themesoporous products do not form by the aggregation of preexistingmesoporous particles. Precursor solutions are clear, typically stablefor several days, and eventually gel rather than form particles, as inthe acid-route synthesis of Huo et al. Chem. Mater. 1994, 6, 1176-1191.

Powder X-ray diffraction (PXRD) patterns of the dried and calcinedfibers are shown in FIG. 10. The (100), (110), (200), and (210)reflections corresponding to a hexagonal structure are visible in thePXRD pattern for the dried fibers, although only the first threereflections are visible for the calcined fibers. The increase in peakintensity after calcination (note scale indicated on FIG. 10) is due tothe greater scattering density contrast and reduced X-ray absorbanceafter surfactant and polymer removal. The increase is not due toenhancement of pore ordering. To the contrary, the loss of the (210)reflection, along with a peak-width increase of the (100) reflection,indicates partial loss of order. The d₁₀₀ value of the fibers decreasedfrom 39 Å to 30 Å on calcination, a decrease comparable to the measuredlinear shrinkage of 25%.

By nitrogen-adsorption analysis, the mesoporous fibers have a surfacearea of 1100 m²/g and a 20 Å pore diameter (see FIGS. 11,12). Though thepore size was smaller than that of MCM-41 materials, because ofcalcination shrinkage, the total surface area is comparable. Theadsorption/desorption isotherms showed no hysteresis within theresolution of the equipment, indicating that the pores wereunconstricted. The hydrophilic polymer is presumably dispersed withinthe silica phase (no crystalline XRD peaks are observed in the driedfibers). However, there appears to be no residual porosity frompyrolysis of the polymer; the adsorption data showed no evidence of amicropore contribution. The volume fraction of mesoporosity was 54%,which correlates well with the 57 vol % porosity calculated from thevolume contributions of the surfactant and silica phases, after takinginto account the volumetric shrinkage measured by the shift in the d₁₀₀peak. We have shown previously that the pore volume fraction inmesoporous films can be controlled by varying the CTAC/TEOS mole ratioin the precursor solution [8]. Raman spectroscopy of calcined fibersindicate that the fibers consist of meta-silicate, consist for silicawith hydroxide terminated surfaces.

The as-spun, dried and calcined fibers all showed birefringence betweencross polarizers in an optical microscope. Consistent with poreorientation along the fiber axis, maximum light transmission occurredwith the fiber 45° to the polarizers and nearly complete extinctionoccurs when parallel and perpendicular to the analyzer. Transmissionelectron microscopy of microtome sections showed pore alignment over alength scale of at least 10 μm. However, because of difficulties in themicrotome technique, the absolute pore orientation with respect to thefiber axis could not be established.

EXAMPLE 7

Spray-Dried Mesoporous Powders

An experiment was conducted to demonstrate making well-orderedmesoporous powders of high surface areas by the method of the presentinvention.

The spray-drying solution was formed by combining deionized water, HCl,CTAC, followed by TEOS to obtain final mole ratios of: 10.0 H₂O, 0.050HCl, 0.12 to 0.28 CTAC, 1.0 TEOS. The solution was mixed to promote thehydrolysis where the surfactant acts as a emulsifying agent to combinethe aqueous and alkoxide phase. In the solution formulation, waterrather than alcohol dilution is used to avoid possible explosionhazards. Solutions were spray-dried in a Buchi 190 Mini Spray Dryeroperating with an outlet temperature of 120° C. Powders were collectedunder a cyclone and calcined under the same conditions as the fibers.

Samples were analyzed by powder X-ray diffraction and by nitrogensorption as described in Example 7 above.

In spray drying the particle morphology was dependent on the precursorsolution composition and drying conditions. The surfactant to silicamole ratio was varied between 0.12 and 0.28. SEM analysis showedparticle morphology was similar to that of the hollow spheres (seeExample 3), except the walls had collapsed during drying. Depending onthe spray drying conditions, a range of particle morphologies werepossible from solid spherical particles to collapse particles to hollowparticles.

Pore volume fraction and the surface area as a function of surfactantconcentration are shown in FIG. 13. The pore-size distribution plots forthe highest surfactant ratio sample are shown in FIGS. 14,15,respectively. A multi-point BET, adsorption/desorption analysis wasconsistent with the pore size analysis. A maximum pore volume fractionof 63% (37 vol % silica) and a surface area up to 1770 m²/g was achievedat the highest surfactant concentration. Nitrogen adsorption/desorptioncurves had no hysteresis and indicate a constant pore size of 25 Å forall surfactant concentrations. In the PXRD patterns for theas-synthesized powders in this series, the (100), (110), (200) and (210)reflections corresponding to a hexagonal array were evident. Aftercalcination the (210) reflections were absent (see FIG. 16).Interestingly, the d₁₀₀ values were relatively constant with surfactantconcentration (˜38 Å as synthesized and −32 Å after calcination).

EXAMPLE 8

Spray-Dried Mesoporous Powders

Aluminum was incorporated into the spray dried powders by the additionof aluminum chloride to the precursor solutions.

Aluminum chloride (hexa-hydrated form, Fischer) was combined with d.i.water, hydrochloric acid, CTAC followed by CTAC in the following moleratios 11.3 H2O/TEOS; 0.10 HCl/TEOS; 0.106 CTAC/(AlCl₃+TEOS); theAlCl₃/TEOS ratios were 0.00, 0.031, 0.063, 0.125 and 0.25. Solutionswere spray-dried in a Buchi 190 Mini Spray Dryer operating with anoutlet temperature of 120° C. Powders were dried overnight at 105° C.and calcined in air at 350° C. for one hour and 600° C. for 1.5 h.

For an aluminum to silica mole ratio of 0.25 (FIG. 17a), the aluminum inthe synthesized powders was a mixture of tetrahedral (framework) andoctahedral. Octahedral aluminum would not impart a negative charge tothe aluminosilicate. For an aluminum to silica mole ratio of 0.063, thealuminum in the as-synthesized powders was predominately framework, asdetermined by ²⁷Al-NMR (see FIG. 17c). However, the frameworksubstitution was not stable with calcination (see FIG. 17d). For analuminum to silica mole ratio of 0.031, the aluminum in the calcinedpowders was roughly two-thirds tetrahedral (framework) and one-third inan octahedral coordination (see FIG. 17b).

The x-ray diffraction patterns for the mesoporous powders with aluminumaddition are shown in FIGS. 18 and 19 for the as-synthesized andcalcined powders. The d-spacing of the primary diffraction peak for eachpowder, before and after calcination, is summarized in Table 8-1. Thetable shows an initial decrease in the d-spacing with addition ofaluminum chloride salt. The calcined powders with Al:Si mole ratios of0.125 and 0.25 had a slight brown color (or tan) indicating incompletecalcination of organics due to limited pore accessibility. The remainingcalcined powders were white.

TABLE 8-1 Sample No. Al:Si molar ratio d₁₀₀ before calc. d₁₀₀ aftercalc. 56314-112A 0.00 36.Å 32.Å 56314-112B 0.031 34.Å 30.Å 56314-112C0.063 34.Å 29.Å 56314-112D 0.125 34.Å 29.Å 56314-112E 0.250 35.Å 31.Å

EXAMPLE 9

Loading of a Catalytically Active Metal into Mesoporous Fibers

An experiment was conducted to demonstrate loading of a catalyticallyactive metal into the mesoporous fibers which were produced by themethod of the present invention.

A mesoporous fiber supported rhodium catalyst was prepared by incipientwetness impregnation of the mesoporous silica fibers with a rhodium(III) nitrate solution. Rhodium is a good catalyst for reactionsincluding methanol decomposition, alkane partial oxidation and fuelcombustion. The loading for rhodium metal was 5% by weight. A solutionof rhodium (III) nitrate (10 wt % assay, in nitric acid, Engelhard) wasdiluted with d.i. water in a volumetric cylinder until the 1.5 ml indexwas reached. A mass of 2.07 g or the mesoporous silica fibers was usedin the catalyst loading. The mesoporous silica fibers were tumbled andthe rhodium (III) nitrate solution was added drop wise. Once the rhodium(III) nitrate was impregnated on the mesoporous silica fibers, thefibers were dried at 100° C. in a vacuum overnight, followed bycalcination at 350° C. for at least one hour. Prior to catalyst testingmesoporous silica fiber supported rhodium catalyst were activated(reduced to metallic rhodium) with a mixed gas of 10% hydrogen and 90%helium (by volume) at 120° C. for at least one hour.

Fibers with 5 wt % rhodium were used as a supported catalyst to convertmethane and air to hydrogen and carbon monoxide. The amount of methanewas 29.5 vol % and the amount of air was 70.5 vol %. A small amount ofrhodium/fiber supported catalyst (0.041 cm³) was used. Residence timewas 8 milli-seconds and the reaction was carried out separately at twotemperatures 360 and 445° C. On day 1, the supported catalyst showedactivity for both temperatures. The supported catalyst was permitted tocool overnight. However, on day 2, the supported catalyst showed noactivity. Because the amount of catalyst was too small to perform ananalysis of the failure, a second series of tests were done with anincreased quantity of fiber supported catalyst (0.442 cm³).

The residence time for the second series was 50 milliseconds. Thesupported catalyst was run at 400° C. Again, the supported catalyst waspermitted to cool overnight. No loss of activity was observed on thesecond day.

Because the failure was not duplicated, the reason for it was notdetermined, nor was the reason for the subsequent success determined.

EXAMPLE 10

An experiment was performed to demonstrate the coating of mesoporoussilica onto glass cover slips. The precursor solutions were modifiedfrom Example 1 by reducing the amount of acid by half. The calcinationtemperature was also lowered to 450° C.

Microscope-slide-cover-slipe substrates (22×22 mm², 0.13 to 0.16 mmthick) were soaked in a solution of sulfuric acid and Nochromix (GodaxLabs) and rinsed with deionized water. The precursor solutions wereprepared in a 30 ml glass bottle. The bottle was rinsed and dried toremove particulates. Reagents were added by mass using disposabletransfer pipettes. Spin-coating precursor solutions were prepared bycombining cetyltrimethylammonium chloride (CTAC) (T.C.I. America),deionized water, ethanol (punctilious; Quantum Chemicals), hydrochloricacid (Mallinckrodt) and tetraethyl orthosilicate (TEOS) (Aldrich). Thesurfactant, water, ethanol and acid were mixed together to allow thesurfactant to completely dissolve before TEOS was added. Mass amounts ofeach reagent in the preparation are shown in Tables E-10a and E-10b fortwo separate formulations. In the second formulation (# 56483-5), theamount of ethanol was reduced approximately by half.

TABLE E-10a Formulation #56483-2 Reagent Mass/g Molar Ratio CTAC 0.8250.11 water 2.903 7.02 ethanol 5.501 5.07 hydrochloric acid (38 wt %)0.122 0.05 TEOS 4.901 1.00

TABLE E-10b Formulation #56483-5 Reagent Mass/g Molar Ratio CTAC 0.8310.11 water 2.934 6.95 ethanol 2.715 2.44 hydrochloric acid (38 wt %)0.148 0.06 TEOS 5.033 1.00

After TEOS hydrolysis (indicated by the exothermic reaction), thesolutions were aged for 1 h prior to coating. The silica species withinthe precursor solutions would be expected to change with aging. Thoughthese have not be characterized, it was found that the XRD peakintensities improved after 1 h aging.

The substrates were flooded with spin-coating solutions and spun 3000rpm with a Specialty Coating System Model P-6204A, using the maximumacceleration setting (spin-up time <1 s). To increase silicacondensation, coated substrates were post-treated by exposing films tothe vapors from drops of concentrated ammonia under an inverted beakerfor about 15 min, followed heating at 105° C. overnight in air andcalcination at 450° C. To prevent cracking of glass substrates, sampleswere placed in the box furnace prior to bringing up to temperature.

In the XRD patterns (see FIGS. 20 and 21), the (100) and (200)reflections are apparent. The absence of the (110) reflection (or otherhigher order peaks) suggests the (100) family of planes of the hexagonalarray are parallel to the substrate surface. The width of the (100)reflection does not change significantly with calcination, indicatingthe good stability of the mesoporous structure. Because of thedifficulty in located the sample in the same position in the X-raydiffractometer before and after calcination, no interpretation can bemade on the change in the peak height (i.e., the decrease of the peakheight in FIG. 20 does not necessarily indicate a loss of structuralorder. The d₁₀₀-spacings are 37.5 Å before and 34.5 Å after calcinationfor the film prepared with formulation #56483-2; and 36.5 and 33 Å,respectively, for calcination for the film prepared with formulation#56483-5.

The top surface of the spin-coater chuck, on which the covers slips sitsduring spin coating, consists of a series of radial groves approximately1 mm apart. This pattern of groves also appears in the interferencecolors of the films spun from both precursor solutions. It is believedthat the chuck acts as a heat sink for the thin cover slip. As thesolvent evaporates during spin coating, spatial variations in heattransfer through the cover slip results in temperature variations whichin turn results in variations in the film thickness.

Closure

While a preferred embodiment of the present invention has been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A method of making mesoporous silica materials, comprising the stepsof (a) combining a silica precursor with an aqueous solvent, an acid anda surfactant having an ammonium cation into a silica precursor solution,(b) templating the silica precursor with the surfactant and obtainingthe mesoporous material from the templated silica precursor, (c) formingsaid silica precursor solution into a preform; and (d) rapidlyevaporating said aqueous solvent from said preform for obtaining themesoporous material, wherein the improvement comprises: (i) providingsaid aqueous solvent in an amount resulting in complete hydrolysis andproviding said acid in an amount maintaining a hydrolyzed precursor andavoiding gelation or precipitation; and (ii) providing said surfactantand said silica precursor in a mole ratio that is above a lower moleratio that produces a non-porous silica phase and below an upper moleratio that produces a lamellar phase.
 2. The method as recited in claim1, wherein said lower mole ratio is about 0.05.
 3. The method as recitedin claim 1, wherein said upper mole ratio is about 0.3.
 4. The method asrecited in claim 1, wherein said acid is added in an amount resulting ina pH of said silica precursor solution of from about 1 to about
 4. 5.The method as recited in claim 4, wherein said pH is about
 2. 6. Themethod as recited in claim 1, wherein the step of forming includesdiluting with an alcohol.
 7. The method as recited in claim 6, whereinsaid alcohol is ethanol.
 8. The method as recited in claim 1, whereinsaid aqueous solvent, said acid, and said surfactant are premixed beforecombining with said silica precursor.
 9. The method as recited in claim1, wherein said mesoporous material is in a geometric form selected fromthe group consisting of fiber, powder, and film.
 10. The method asrecited in claim 1, wherein said forming is spin-casting.
 11. The methodas recited in claim 1, wherein said forming is spraying.
 12. The methodas recited in claim 1, further comprising adding a pre-polymer or apolymer to said silica precursor solution making a pituitous mixture.13. The method as recited in claim 1, wherein said forming is drawing.14. The method as recited in claim 1, wherein said forming issqueegeeing.
 15. The method as recited in claim 1, further comprisingthe step of adding a metal compound to the silica precursor solution.16. The method as recited in claim 15, wherein said metal compound isselected from the group consisting of metal halide, metal nitrate, andcombinations thereof.
 17. The method as recited in claim 16, whereinsaid metal halide is a metal chloride.
 18. The method as recited inclaim 16, wherein said metal is selected from the group of aluminum,iron and combinations thereof.
 19. The method as recited in claim 1,wherein said silica precursor is an alkoxide silica precursor or atetrachlorosilane.
 20. The method as recited in claim 1, wherein saidaqueous solvent amount is characterized by a ratio of said aqueoussolvent to said silica precursor of about
 7. 21. The method as recitedin claim 1, wherein said acid amount is characterized by a ratio of saidacid to said silica precursor of about 0.1.
 22. The method as recited inclaim 1, further comprising adding a swelling agent to the silicaprecursor solution.
 23. The method as recited in claim 22, wherein saidswelling agent is 1,3,5-thimethylbenzene.
 24. The method as recited inclaim 1, further comprising the step of calcining the mesoporousmaterial.
 25. A method of making a mesoporous silica film, comprisingthe steps of (a) combining a silica precursor with an aqueous solvent,an acid and a surfactant having an ammonium cation into a silicaprecursor solution, (b) templating the silica precursor with thesurfactant and obtaining the mesoporous material from the templatedsilica precursor, (c) forming said silica precursor solution into apreform; and (d) rapidly evaporating said aqueous solvent from saidpreform for obtaining the mesoporous material, wherein the improvementcomprises: (i) said silica precursor is tetraethoxysilane; (ii)providing said aqueous solvent in a superstoichiometric amount andproviding said acid in an amount maintaining a hydrolyzed precursor andavoiding gelation or precipitation; (iii) providing said surfactant andsaid silica precursor in a mole ratio that is above a lower mole ratiothat produces a non-porous silica phase and below an upper mole ratiothat produces a lamellar phase; and (iv) said forming includes dilutingwith an alcohol.
 26. The method as recited in claim 26, furthercomprising adding a pre-polymer or a polymer to said silica precursorsolution making a pituitous mixture.
 27. The method as recited in claim26, wherein said rapidly evaporating is by spin-casting.
 28. A method ofmaking mesoporous silica materials, comprising the steps of (a)combining a silica precursor with an aqueous solvent, an acid and asurfactant having an ammonium cation into a silica precursor solution,(b) templating the silica precursor with the surfactant and obtainingthe mesoporous material from the templated silica precursor, (c) formingsaid silica precursor solution into a preform; and (d) rapidlyevaporating said aqueous solvent from said preform for obtaining themesoporous material, wherein the improvement comprises: (i) providingsaid aqueous solvent in an amount resulting in complete hydrolysis andproviding said acid in an amount maintaining a hydrolyzed precursor andavoiding gelation or precipitation; and (ii) providing said surfactantand said silica precursor are in a mole ratio that is above a lower moleratio that produces a non-porous silica phase and below an upper moleratio that produces a lamellar phase.
 29. The method as recited in claim28, wherein said lower mole ratio is about 0.05.
 30. The method asrecited in claim 28, wherein said upper mole ratio is about 0.3.
 31. Themethod as recited in claim 28, wherein said acid is added in an amountresulting in a pH of said silica precursor solution of from about 1 toabout
 4. 32. The method as recited in claim 31 wherein said pH is about2.
 33. The method as recited in claim 28, wherein the step of formingincludes diluting with an alcohol.
 34. The method as recited in claim33, wherein said alcohol is ethanol.
 35. The method as recited in claim28, wherein said aqueous solvent, said acid, and said surfactant arepremixed before combining with said silica precursor.
 36. The method asrecited in claim 28, wherein said mesoporous material is in a geometricform selected from the group consisting of fiber, powder, and film. 37.The method as recited in claim 28, wherein said forming is spin-casting.38. The method as recited in claim 28, wherein said forming is spraying.39. The method as recited in claim 28, further comprising adding apre-polymer or a polymer to said silica precursor solution making apituitous mixture.
 40. The method as recited in claim 28, wherein saidforming is drawing.
 41. The method as recited in claim 28, wherein saidforming is squeegeeing.
 42. The method as recited in claim 28, furthercomprising the step of adding a metal compound to the silica precursorsolution.
 43. The method as recited in claim 42, wherein said metalcompound is selected from the group consisting of metal halide, metalnitrate, and combinations thereof.
 44. The method as recited in claim43, wherein said metal halide is a metal chloride.
 45. The method asrecited in claim 43, wherein said metal is selected from the group ofaluminum, iron and combinations thereof.
 46. The method as recited inclaim 28, wherein said silica precursor is an alkoxide silica precursoror a tetrachlorosilane.
 47. The method as recited in claim 28, whereinsaid aqueous solvent amount is characterized by a ratio of said aqueoussolvent to said silica precursor of about
 7. 48. The method as recitedin claim 28, wherein said acid amount is characterized by a ratio ofsaid acid to said silica precursor of about 0.1.
 49. The method asrecited in claim 28, further comprising adding a swelling agent to thesilica precursor solution.
 50. The method as recited in claim 49,wherein said swelling agent is 1,3,5-thimethylbenzene.
 51. The method asrecited in claim 28, further comprising the step of calcining themesoporous material.
 52. A method of making a mesoporous silica film,comprising the steps of (a) combining a silica precursor with an aqueoussolvent, an acid and a surfactant having an ammonium cation into asilica precursor solution, (b) templating the silica precursor with thesurfactant and obtaining the mesoporous material from the templatedsilica precursor, (c) forming said silica precursor solution into apreform; and (d) rapidly evaporating said aqueous solvent from saidpreform for obtaining the mesoporous material, wherein the improvementcomprises: (i) said silica precursor is tetraethoxysilane; (ii)providing said aqueous solvent in a superstoichiometric amount andproviding said acid in an amount maintaining a hydrolyzed precursor andavoiding gelation or precipitation; (iii) providing said surfactant andsaid silica precursor in a mole ratio that is above a lower mole ratiothat produces a non-porous silica phase and below and upper mole ratiothat produces a lamellar phase; and (iv) said forming includes dilutingwith an alcohol.
 53. The method as recited in claim 52, furthercomprising adding a pre-polymer or a polymer to said silica precursorsolution making a pituitous mixture.
 54. The method as recited in claim52, wherein said rapidly evaporating is by spin-casting.
 55. A method ofmaking a mesoporous film on a substrate, the method comprising the stepsof: (a) combining a silica precursor with an aqueous solvent, an acidcatalyst and an ammonium cationic surfactant into a precursor solution;(b) dispensing said precursor solution onto the substrate; (c) forming afilm by evaporation of the solvent in less than 5 minutes; and (d)heating the film on the substrate to a temperature sufficient todecompose the surfactant, thereby producing a mesoporous film on thesubstrate.
 56. The method of claim 55 wherein the precursor solution isa silica precursor solution and wherein the surfactant and the silicaprecursor solution are in a mole ratio that is above a lower mole ratiothat produces a non-mesoporous silica phase and below an upper moleratio that produces a lamellar phase.
 57. The process of claim 55,wherein the film exhibits an index of refraction between 1.16 and thatof silica.
 58. A process to form mesostructured films, comprising: (a)preparing a precursor sol containing a soluble source of silica, anaqueous solvent, an ammonium cationic surfactant and an acid catalyst;and (b) depositing the precursor sol on a substrate wherein evaporationof solvent and water is less than 5 minutes causes the formation of saidmesostructured films on the substrate surface.
 59. The process of claim58 wherein the aqueous solvent and the catalyst are provided in amountsthat maintain a hydrolyzed precursor sol while avoiding gelation orprecipitation.
 60. The process of claim 58 wherein the soluble source ofsilica is a silica precursor alkoxide or tetrachlorosilane and whereinthe surfactant and the soluble source of silica are in a mole ratio thatis above a lower mole ratio that produces a non-porous silica phase andbelow an upper mole ratio that produces a lamellar phase.
 61. Theprocess of claim 58, wherein the ammonium cationic surfactant furtherincludes alkyl triethylammonium chloride or bromide surfactants withdifferent chain lengths.
 62. The process of claim 58, further comprisingthe step of calcining said film at 450° C.
 63. The process of claim 58,wherein the precursor sol is deposited on a substrate by spin coating.64. The process of claim 58, wherein said soluble source of silica is analkoxide silica precursor or tetrachlorosilane.
 65. The process of claim58, wherein the films exhibit an index of refraction between 1.16 andthat of silica.
 66. A process to form a mesoporous structure,comprising: (a) preparing a precursor sol containing a soluble source ofsilica, an alcohol and water solvent, an ammonium cationic surfactant,and an acid catalyst, wherein said solvent is provided in an amountresulting in complete hydrolysis and said acid catalyst is in an amountto maintain a hydrolyzed precursor and to avoid gelation orprecipitation in said precursor sol; (b) forming the precursor sol intoa preform; (c) evaporating said solvent from the preform at a rate thatforms a mesostructured material; and (d) calcining the mesostructuredmaterial to form a mesoporous structure.
 67. The process of claim 66,wherein said precursor sol contains alcohol which is a byproduct ofhydrolysis, and said mesoporous structure is a film.
 68. The process ofclaim 66, wherein said preform is a droplet, said alcohol is a byproductof hydrolysis, and said sol is spray dried to form a powder.
 69. Theprocess of claim 66, wherein said drying is preformed in less than 5minutes.
 70. The process of claim 66, wherein said precursor solcontains dilutant alcohol, and wherein the mesoporous structure is afilm.
 71. The process of claim 66, wherein the mesoporous structure is afilm and wherein the film exhibits an index of refraction of between1.16 and that of silica.
 72. The process of claim 66, wherein the saidprecursor sol contains alcohol which is a byproduct of hydrolysis, andwherein said mesostructure is a film.
 73. The process of claim 72,wherein the film exhibits an index of refraction of between 1.16 andthat of silica.
 74. The process of claim 66, wherein said preform is adroplet, wherein said alcohol is a byproduct of hydrolysis, and whereinsaid precursor sol is spray dried.
 75. The process of claim 66, whereinsaid evaporating is performed in less than 5 minutes.
 76. The process ofclaim 66, wherein said soluble source of silica includes a silicaalkoxide precursor or tetrachlorosilane.
 77. A process to form amesoporous structure, comprising: (a) preparing a precursor solcontaining a soluble source of silica, an alcohol and water solvent, anammonium cationic surfactant, and an acid catalyst, wherein said solventis provided in an amount resulting in complete hydrolysis and said acidis in amount to maintain a hydrolyzed precursor and to avoid gelation orprecipitation in said precursor sol; (b) forming the precursor sol intoa preform; (c) evaporating said solvent from the preform at a rate thatforms a mesostructured material, wherein said mesostructured materialcontains surfactant; and (d) calcining the mesostructured material toform a mesoporous structure.
 78. A process to form a mesostructure,comprising: (a) preparing a precursor sol containing a soluble source ofsilica, water and alcohol solvent, an ammonium cationic surfactant andan acid catalyst; and (b) evaporating said solvent in less than 5minutes to cause the formation of a mesostructure, wherein saidmesostructure contains surfactant.
 79. The process of claim 78, whereinthe mesostructure is a film, and wherein the film exhibits an index ofrefraction of between 1.16 and that of silica.
 80. A process to form amesostructure, comprising: (a) preparing a precursor sol containing asoluble source of silica, a water and alcohol solvent, an ammoniumcationic surfactant and an acid catalyst, and (b) evaporating saidsolvent in less than 5 minutes to cause the formation of amesostructure.
 81. The process of claim 80, wherein said solvent isevaporated in less than 1 minute.
 82. The process of claim 80, whereinsaid solvent is evaporated in less than 10 seconds.
 83. The process ofclaim 82, wherein the mesostructure is a film, and wherein the filmexhibits an index of refraction of between 1.16 and that of silica. 84.The process of claim 80, wherein the said precursor sol contains bothdilutant alcohol and alcohol which is a byproduct of hydrolysis, andwherein said mesostructure is a film.
 85. The process of claim 80,wherein said preform is a droplet, said alcohol is a byproduct ofhydrolysis, and said sol is spray dried.
 86. The process of claim 80,wherein the ammonium cationic surfactant further includes alkyltriethylammonium chloride or bromide surfactants with different chainlengths.